Focus detection methods conventionally used in digital cameras are, for example, a contrast detection method used in video cameras or compact cameras, and a phase-difference detection method used in single-lens reflex cameras.
A phase-difference detection type focus detection method adopted in single-lens reflex cameras is disclosed in, for example, Japanese Patent Laid-Open No. 2-120712. A single-lens reflex camera includes a quick return mirror to guide object light to a viewfinder optical system so that the user can observe the object via an imaging lens. The quick return mirror is formed from a half mirror which transmits part of light. Light having passed through the quick return mirror is guided to a phase-difference detection type focus detection device to detect the focus of the imaging lens. In photographing with a photosensitive medium such as a film or image sensor, the quick return mirror retracts from the photographic optical path. During photographing with a photosensitive medium, no conventional phase-difference detection type focus detection can be done.
A contrast detection type focus adjustment method is generally used to observe or record a moving image in a camera having an image sensor as a photosensitive medium. For example, Japanese Patent Laid-Open No. 63-127217 discloses the focus detection method. According to the contrast detection type focus adjustment method, while moving the focus lens of an imaging lens along the optical axis, the high-frequency component of an image sensed by an image sensor is extracted. A lens position where the contrast peaks is detected to adjust the focus. This focus adjustment method compares the contrasts of images while moving the focus lens, and cannot adjust the focus quickly.
Under the circumstances, the present applicant has applied Japanese Patent Laid-Open No. 2009-103997 which discloses a technique capable of phase difference-based focus detection by restricting a light-receivable area in some pixels forming an image sensor.
In the technique disclosed in Japanese Patent Laid-Open No. 2009-103997, some pixels of the image sensor are dedicated to focus detection. In a normal captured image, the focus detection pixels become defective pixels, degrading the image quality.